Interpretive Summary: Pesticide application is complicated by the use of a variety of delivery equipment and methods, the varied physical properties of chemical sprays, the diverse crops and their growth habits, the numerous pests and diseases, the disparity of operator skills, the uncontrollable weather conditions, the extensive worker safety and environmental regulations, and the economics related to the benefits of pesticide applications. During the past several decades, research into pesticide spray application technology has mostly concentrated on methods and equipment to improve the accuracy of spray delivery to targets. However, there are few studies on the behaviour of droplets deposited onto targets following delivery. The behaviour of droplets on plant surfaces contributes to controlling the physiological and biological processes. The leaf surfaces attacked by pests vary with the type of insect or disease. Different insects have their favourite feeding locations on leaves. This research determined the evaporation time, droplet spreading process and spreading area of droplets with and without the non-ionic surfactant at different positions (i.e. interveinal area, secondary vein, and midrib) on adaxial and abaxial surfaces of waxy leaves. Results from this research will provide quantitative information for end users to increase pesticide application efficiency for controlling particular insects or diseases.

Technical Abstract:
The evaporation and formation of deposit patterns from single droplets deposited at various locations on waxy leaves were investigated under controlled conditions. Leaf locations included the interveinal area, midrib and secondary vein on both adaxial and abaxial surfaces. Tests were conducted with 300 and 600 µm diameter droplets containing water and a non-ionic surfactant. The ambient temperature was 25ºC and relative humidity was 60%. Evaporation time and wetted area varied with the area where the droplets were deposited on the leaf surfaces. The variation in evaporation time for 300 µm diameter droplets without surfactant on the interveinal area, midrib and secondary vein of adaxial surface was 30% whilst the variation of wetted area was 39%. The wetted area was significantly larger on the adaxial surface than on the abaxial surface but the evaporation time between both surfaces was not significantly different. For the whole leaf, the average evaporation time of 300 µm diameter droplets decreased by 44% and the average wetted area increased by 202% when 0.25% non-ionic surfactant was added to the spray solution. The total mean evaporation time increased 279% and the wetted area increased 166% without the surfactant, 452% and 229% with the surfactant when droplet diameter was increased from 300 to 600 µm. The largest deposit area was measured on the midrib of adaxial surface with added surfactant. The 300 µm diameter droplets had longer evaporation times per droplet volume and greater wetted area per droplet volume than the 600 µm diameter droplets, thereby supporting the hypothesis that increased pesticide application efficiency could be achieved by finer spray. This study also demonstrated that the ratio between spray coverage area and the amount of spray liquid required could be increased by the use of surfactants, thereby offering possibilities of reduction in spray application rates and of increase in application efficiency.